The bright pigments are used widely for various applications, such as automobiles, motorcycles, OA (office automation) machines, cellular phones and home electric appliances, printing matters and writing tools, for the purpose of improving the cosmetic appearance thereof.
As bright pigments, there have been conventionally known mica flakes and graphite flakes each coated with metal oxides such as titanium dioxide and iron oxide, iron oxide particles composed mainly of alpha-iron oxide crystals, and the like. Such a bright pigment reflects light incident from the outside upon its surface and shines brilliantly. Therefore, the bright pigment gives an unique, varied and excellent cosmetic appearance, when contained in a paint, to the coated surfaces, when contained in an ink, to the drawn lines or the printed surfaces, or when contained in a resin composition, to the surfaces of resin molded product, in synergy with the color tones of each of the base.
Among pearl colors, for example, a so-called white pearl color with high whiteness is popular particularly for coating luxury automobiles. The coating with a white pearl color can be achieved generally by a multilayer coating film formed of three films, that is, a base-coat film containing a titanium dioxide pigment, a metallic base-coat film containing a bright pigment with a pearly luster in which flaky substrates each are coated with titanium dioxide, and a top clear-coat film.
As one of the causes that impair the fine appearance of the coating with a white pearl color, faint yellow in the titanium dioxide pigment and yellow turbidity due to the complementary color of the interference color derived from the bright pigment can be mentioned. Bright pigments to be used for paints for automobiles and motorcycles for outdoor use may include a coating layer containing the hydroxide or hydrated oxide of cerium for preventing the photocatalytic activity of the titanium dioxide. There is, however, a problem that the hydroxide or hydrated oxide of cerium is likely to cause yellow turbidity. As to such a coating layer containing the hydroxide or hydrated oxide of cerium, the following techniques are conventionally known.
JP 60(1985)-92359 A, for example, discloses a pigment in which polysiloxane and an oxide or hydroxide of cerium are deposited on mica flakes coated with a metal oxide.
JP 1(1989)-292067 A discloses a pigment in which the following hydrates (i) and (ii) are deposited on the surface of mica flakes coated with a metal oxide. The hydrates are: (i) a hydrated zirconium oxide produced by hydrolysis of a zirconium compound in the presence of hypophosphorous acid; and (ii) a hydrated oxide produced by hydrolysis of a water-soluble compound of at least one metal selected from the group consisting of cobalt, manganese, and cerium.
JP 7(1995)-18199 A discloses a pigment in which a sheet-shaped fine substrate coated with a metal oxide is coated with a covering layer. The covering layer contains silicon dioxide, the hydroxide or hydrated oxide of cerium, aluminum or zirconium, and an organic coupling agent.
JP 1(1989)-313575 A discloses use of a fluorescent whitening agent as a countermeasure to the yellow turbidity in the coating with a white pearl color.
JP 11(1999)-130975 A discloses a composition of a fluorescent organic white pigment that has an excellent masking effect, as a substitute for the titanium dioxide pigment to be used for white base-coat films.
JP 2006-192384 A discloses a bright base-coat film containing a bright pigment and a fluorescent white pigment, in a multilayer coating film formed by subsequently forming a colored base-coat film, a bright base-coat film and clear-coat film on a substrate.
These fluorescent whitening agents absorb ultraviolet rays (with a wavelength of around 380 nm) and emit blue fluorescence (with a wavelength of around 440 nm). For this reason, when they are used in a yellowish coating film, blue luminescence occurs additionally therein, thereby producing the effect of a shiny white appearance on the film.
As another countermeasure to the yellow turbidity in the coating with a white pearl color, JP 2004-275972 A discloses a method for forming a multilayer coating film in which a color base paint containing a colored pigment, a mica base paint containing an interference mica pigment and a clear paint are subsequently applied onto a base material formed with an undercoat film and a middle coat film. The above-mentioned mica base paint, as well as the interference mica pigment, contains titanium oxide fine particles with an average particle size of 0.01 to 0.1 μm in which the surface of each particle is covered by a coating layer composed of alumina and/or zirconia formed thereon.
However, the yellow turbidity in the coating film is due to the bright pigment whose hue varies depending on the angle of view, and therefore the use of fluorescent whitening agents or titanium oxide fine particles does not have a significant effect on the reduction of the yellow turbidity. Rather, fluorescent whitening agents suffer from problems, such as a rapid reduction of the whitening effect in outdoor use with time because of their poor weather resistance, and a quantitative limitation in use because of concentration quenching. The titanium oxide fine particles also are problematic because they scatter the reflected light from the bright pigment and reduce the brightness of the bright pigment.
On the other hand, among metallic colors, a so-called metallic silver color with good shadow quality is popular for coating sports sedans, because it makes the style of an automobile distinctive. The coating with a metallic silver color can be achieved by a multilayer coating film formed of two films, that is, a color metallic base-coat film and a top clear-coat film. The color metallic base-coat film often contains a colored pigment of carbon and titanium oxide, and aluminum flakes. In order to improve the shadow quality, there is an attempt in which flaky glass coated with silver having a high reflectance of visible light is used as a bright pigment instead of the aluminum flakes.
However, the silver-containing coating layer has a high reflectance on the long wavelength side (more than 500 nm and 700 nm or less), and has a low reflectance on the short wavelength side (500 nm or less). Therefore, yellow turbidity is likely to appear also in the coating with a metallic silver color that includes a bright pigment having a silver-containing coating layer. These problems also are mentioned in JP 2001-72933 A.